Mouse button 3 sometimes acts as a 'Back' button

Sometimes when clicking mouse button 3, it acts as a 'Back' button (i.e. it navigates to wherever I was just before). However, this doesn't happen most of the time; it's fairly rare instead. Sometimes when I open Discord from being minimized to tray, every mouse 3 click acts as a 'Back' button, but usually all mouse 3 clicks act normally (i.e. don't do anything). Unfortunately I have no idea what causes this. I thought it might be because of waking up from sleep mode but that didn't seem to do the trick.

Just for the record, for me mouse button 3 is the left thumb button on the mouse, and mouse button 1 is the right mouse button and mouse button 2 is the scroll wheel.

Anyway, this is fairly annoying because it's not at all predictable. I also happen to have mouse 3 bound as my push-to-talk key, so there's that.